Anaerobic Lagoons (ALs) are typically earthen basins filled with wastewater, which undergoes anaerobic respiration as part of a system designed to manage and treat wastewater from poultry slaughter houses. Once in the lagoon, wastewater undergoes, among other reactions, the process of anaerobic respiration, whereby volatile organic compounds are converted into carbon dioxide and methane. ALs are typically operated on a five day per week schedule, with little or no production of wastewater over weekends and on holidays.
Anaerobic Lagoons are designed to provide BOD, Total Suspended Solids (TSS), and Fat, Oil and Grease (FOG) removal. The anaerobic biological treatment process typically provides approximately 70% to 90% BOD removal, 80% to 90% FOG removal, and reduces TSS concentrations down to 100 to 300 mg/L. By contrast, the AL process provides only minimal TKN and Total Phosphorous (TP) nutrient removal of about 5% to 15%. Consequently, a typical AL design providing 80% BOD and 10% TKN removal results in AL effluent with a BOD/TKN ratio below 1. However, a BOD/TKN of 3.0 or higher is normally required to achieve efficient nitrate nitrogen removal by biological denitrification in a downstream activated sludge treatment process. Accordingly, existing AL technology is problematic in that ALs remove most of the BOD to generate biogas but remove very little nitrogen causing the typically very low AL effluent BOD/TKN ratio.
More recent AL designs have used influent short circuiting or partial AL bypassing to attempt to raise the AL effluent BOD concentration and BOD/TKN ratio of the AL effluent wastewater or the downstream process blend effluent wastewater. Influent short circuiting is accomplished by use of an alternate AL influent pipeline, which can discharge a controlled portion of the total influent flow volume into the outlet end of the AL in order to allow for FOG and TSS removal from the short circuited portion of the wastewater while significantly reducing the removal of soluble and colloidal BOD from the short circuited wastewater. Influent short circuiting produces a treated wastewater blend that has a higher BOD concentration, and, therefore, a higher BOD/TKN ratio in order to provide an adequate BOD carbon source for efficient biological denitrification in a downstream activated sludge treatment process.
The short circuit design or the partial bypass design, however, are not always effective, especially on weekends or holidays when a processing plant is typically not in operation, and, therefore there is no wastewater to short-circuit to the outlet end of the AL or to bypass the AL. Consequently, even when the short circuit AL lagoon design or AL lagoon bypass design is used, the blended AL effluent wastewater BOD is significantly decreased on weekends and holidays, when there is no influent wastewater flow because the processing plant is not in operation.